Lemon powered calculator!

In this project we are going to build a lemon battery, then use this to power a calculator. Many thanks to Jess for helping me with this one.


With a zinc and a copper electrode stuck into either side of a juicy lemon you can generate about 0.9 volts. Connect a couple of these lemon 'cells' together and you can make a battery that can generate enough power to supply a watch or a calculator.

What you will need

1. two galvanised screws (must be zinc coated - that's what galvanised means).
2. high current single core copper wire (not the stranded type). My wire was an offcut from Homebase.
3. stranded wire (from the Worcestershire Resource Exchange).
4. a cheap calculator (mine was £1.49 from Staples).
5. 2 lemons. Have a think about going organic. I know they are more expensive but by buying organic you are not promoting the use of nasty chemicals (they do less to them but they are more expensive. Erm...).
   
6. plastic cups (to hold the lemons).
7. piece of wood (to attach everything to).
8. wood screws and cup washers (to do the attaching).

You might also find a voltmeter useful to check the voltage your lemon battery is generating and the polarity (in other words which terminal is positive and which is negative). We also used a soldering iron to connect all the wires together but it isn't vital as long as you've got a good connection at each joint.

How to put it all together

Firstly cut the tops off the plastic cups so that they hold the lemons in place. Screw these onto the wood, nearer what will be the back.

Next cut yourself 2 bare lengths of copper wire about 6 or 7 cm long.

For each lemon, push a screw into one side and a length of copper into the other. The copper will be the positive terminal and the screw the negative terminal (if you have a voltmeter you can check this).

Now you need to connect the negative terminal of one cell to the positive terminal of the other (in other words connect the screw in one lemon to the copper wire in the other). We soldered a length of wire between them, but it really doesn't matter how you join them up as long as you have a good connection.

Now solder a longer wires to the two ends of the battery ready to connect to the calculator.

The calculator we have is solar powered (see the post Charge your batteries using the sun! for more info on solar cells) with a battery backup. We disconnected the solar cell and soldered the wires from our battery onto the calculator's battery terminals (if you see what I mean)...


Now try turning on your calculator! To prove to yourself that it is the lemon battery powering the calculator try pulling out one of the copper electrodes.

How does that work then?

All metals try to either give out or take in electrons. So to create a battery firstly we need two metals - one that gives out electrons and one that takes them in. We used zinc (the coating on the screw) and copper (the wire) because they are pretty easy to come by. If you put the wire and the screw into a liquid that will conduct electricity, technically called an electrolyte (in our case lemon juice), then electrons from the copper (which tries to give out electrons) will move over to the zinc (which tries to take in electrons). Electrons are negatively charged so the copper, which is now short of electrons, ends up being positively charged and the screw, which now has electrons from the copper, ends up being negatively charged.

Try putting a screw on one side of your tongue and a length of copper wire on the other. Does it tingle?